Integrated services digital network (ISDN) switching systems are now being deployed around the world, making new features and services available to customers and providing voice and data communication through a single line into the customer premises. Customer acceptance of ISDN is somewhat slow, due in part to the cost of new equipment needed to use ISDN services. A customer cannot plug an analog telephone into an ISDN line and receive telephone services. Likewise, a customer cannot plug a personal computer or other digital data terminal directly into an ISDN line and receive data services. Part of the problem is the nature of ISDN transmission.
According to ISDN standards as set forth by the International Telegraph and Telephone Consultative Committee (CCITT), ISDN customer premises equipment communicates with ISDN switching systems in two 64 kilobits per second (kbps) channels, referred to as B-channels, and in one 16 kbps per second channel referred to as a D-channel. Each of the B-channels is usable to convey digitized voice samples at the rate of 8,000, 8-bit samples per second or data at a rate of 64 kbps. The D-channel is used both to convey signaling packets to effect message signaling between ISDN stations and to convey data packets among ISDN stations.
One system for transmission between ISDN customer premises equipment and the ISDN switching system uses a 4-wire, digital subscriber line (DSL). A 4-wire DSL conveys a serial bit stream at the rate of 192 kbps, which comprises 144 kbps, for the above-mentioned two 64 kbps B-channels and one 16 kbps D-channel and which further comprises 48 kbps used for a number of functions including framing, DC balancing, control and maintenance. A 4-wire DSL represents what is referred to by International Telegraph and Telephone Consultative Committee (CCITT) as the T-interface.
Customers connecting to the T-interface require either a special telephone, (such as the AT&T 7500 series of telephones), to which the customer may connect the serial port of a personal computer, or a special board in a personal computer, (such as an AT&T Personal Computer Terminal Adapter) to which the customer may connect a standard analog telephone. Both of these alternatives are relatively expensive, require special knowledge to set up, and may require that the telephone network between the ISDN switching system and the customer premises be rewired from a 2-wire to a 4-wire network.
CCITT recommendations also recognize a U-interface, which provides ISDN service using a 2-wire DSL. This 2-wire DSL transmits a serial bit stream at the rate of 80 kbps where each bit conveys four levels of information, yielding an effective data rate of 160 kbps, thus providing two 64 kbps B-channels, one 16 kbps D-channel, and signaling and maintenance information. The advantage of the U-interface is that the local telephone network does not require rewiring. Neither a standard analog telephone nor a digital data terminal can connect directly to a 2-wire DSL. Additionally, a further piece of customer premises equipment, a network interface, is required to provide 2 to 4 wire conversion and the associated data rate conversion. The network interface is then connected to the above-discussed special telephone or special personal computer board.
Therefore, a problem in the art is that there is no inexpensive apparatus for providing a plug-in and use interface to an ISDN digital subscriber line for both analog telephones and digital data terminals.